This invention relates to an apparatus for feeding thread in a sewing machine. In a conventional sewing machine, thread is fed by the reciprocating motion of a thread take-up lever. The length of thread fed is determined by friction from the tension disc in the thread pass. The tension disc is adjusted manually by an operator to match a sewing condition.
By analyzing the sewing mechanism, it has recently been found that the quantity of stitches sewn result in accurate control of the thread length fed to the needle.
In other words, by sensing the thickness of fabric sewn and the pitch of the stitch, an adequate length of thread fed can be calculated, and therefore, automated sewing is achieved.
An apparatus has already been proposed constituting a thread feeding roller which is driven by a stepping motor which is controlled to feed a proper length of thread to the needle for every stitching motion.
However, in this known apparatus, a longer length of thread tends to be fed by inertia, and this tendency is exaggerated in high speed sewing. Furthermore, when thick fabric is sewed, the tension created in the thread is increased which requires the stepping motor having a higher capacity. Therefore, there is a higher cost to manufacture the device, and the device is uneconomical to market.